Optical amplification module and laser light source designed to suppress photodarkening

ABSTRACT

The present invention relates to an optical amplification module having a construction which effectively suppresses photodarkening, and to a laser light source including the same. The laser light source comprises a light source for outputting light to be amplified, and an optical amplification module. The optical amplification module comprises two types of optical amplification media having different rare earth element concentrations, and a pumping light source. The low concentration medium and the high concentration medium are disposed in the propagation direction of pumping light such that the population inversion of the low concentration medium is higher than that of the high concentration medium. Hence, by arranging two types of optical amplification media with different rare earth concentrations such that the population inversion of the low concentration medium is higher than that of the high concentration medium, sufficient overall gain of the laser light source can be obtained while effectively suppressing photodarkening in the two types of optical amplification media.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Provisional Application Ser. No.60/780,074 filed on Mar. 8, 2006 by the same Applicant, which is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical amplification module whichamplifies light in an optical amplification medium, and to a laser lightsource including the optical amplification module.

2. Related Background Art

At present, processing technology using laser beams is attracting muchattention, and demand for laser light sources is increasing in variousfields such as machining and medical treatment. In particular, amongthese laser light sources, a fiber laser light source has beenattracting special attention. This fiber laser source comprises anoptical fiber which is doped with various rare earth elements such asYb, Er and Tm as an optical amplification medium, and therefore thefiber laser can amplify light to be amplified by supplying pumping lightto the optical amplification medium and can produce a laser oscillationby a resonator structure. The advantages of the fiber laser source arethat, since the laser beam is enclosed within the optical fiber,treatment is easy, and since heat radiation properties are good, alarge-scale cooling installation is not required.

[Non-Patent Document 1] CLEO/Europe Conference '05, No. CP2-2-THU, 2005

SUMMARY OF THE INVENTION

The inventors have studied conventional laser light sources in detail,and as a result, have found problems as follows.

Namely, in a conventional laser light source, high power light isgenerated within an optical fiber. The generation of high power lightdamages the dopants and impurities in the optical fiber, and this leadsto increasing loss of the optical fiber itself. This phenomenon iscalled photodarkening, and is due to the fact that the rare earthelement, in the optical amplification medium which have a highpopulation inversion, is damaged by the high power light. With a fiberhaving a high rare earth doping concentration, photodarkening tends tooccur relatively easily. In order to overcome the above-mentionedproblems, it is an object of the present invention to provide an opticalamplification module having a suitable construction for effectivelysuppressing photodarkening, and a laser light source including the same.

In an optical amplification module according to the present invention,there are provided two types of optical amplification media withdifferent rare earth element concentrations, these being disposed suchthat the population inversion of the medium having a low rare earthelement concentration is higher than that of the medium having a highrare earth element concentration. Hence, sufficient amplification of thelaser light source can be obtained overall while photodarkening of thetwo types of optical amplification medium is effectively suppressed.More specifically, the optical amplification module according to thepresent invention comprises first and second optical amplification mediathat differ in doped rare earth element concentration, a first pumpinglight source, and a first optical multiplexer.

The first optical amplification medium has an optical waveguide regionwhich is doped with a rare earth element in a predeterminedconcentration. The first optical amplification medium has a lightentrance end into which light enters, and a light exit end which emits afirst amplified light that has been amplified inside the first opticalamplification medium. On the other hand, the second opticalamplification medium has an optical waveguide region which is doped witha rare earth element in a higher concentration than the rare earthconcentration of the first optical amplification medium, and isconnected to this first optical amplification medium. The second(optical amplification medium has a light entrance end into which thefirst amplified light enters, and a light exit end which emits a secondamplified light that has been amplified inside the second opticalamplification medium. The first pumping light source outputs a firstpumping light of a predetermined wavelength. The first opticalamplification medium and the second optical amplification medium may beconnected together directly by fusion-splicing, or they may be opticallyconnected via optical components such as an isolator and a band passfilter. The first optical multiplexer multiplexes the light to beamplified together with the first pumping light. The first opticalmultiplexer is disposed on the light entrance end side of the firstoptical amplification medium such that the first pumping lightpropagates I the order from the first optical amplification medium tothe second optical amplification medium. A first pumping section isconstituted by the first pumping light source and the first opticalmultiplexer.

In the optical amplification module having the aforesaid construction,the first pumping light outputted from the first pumping light sourcefirst propagates through the first optical amplification medium, andthen through the second optical amplification medium. The incident lightis amplified in the first optical amplification medium, and a firstamplified light is emitted from the first optical amplification medium.This first amplified light is further amplified in the second opticalamplification medium, and a second amplified light is emitted from thesecond optical amplification medium. In the first optical amplificationmedium into which the first pumping light is introduced, although thepower of the first pumping light is relatively high, the rare earthelement concentration is relatively low. However, in the second opticalamplification medium through which the first pumping light subsequentlypropagates, although the rare earth element concentration is relativelyhigh, the power of the first pumping light becomes relatively low. Thisconstruction makes it possible to increase the population inversion ofthe first optical amplification medium with a low rare earth elementconcentration, while at the same time suppressing the populationinversion of the second optical amplification medium with a high rareearth element concentration. Therefore, in both the first opticalamplification medium and second optical amplification medium, a highamplification gain can be obtained while photodarkening is suppressed.

In the optical amplification module according to the present inventionsthe population inversion in the first optical amplification medium ispreferably 40% or more, and the rare earth element concentration in thefirst optical amplification medium is preferably not greater than halfof the rare earth element concentration in the second opticalamplification medium. The unsaturated absorption of the first opticalamplification medium is preferably not less than 60% of the unsaturatedabsorption of the second optical amplification medium. The populationinversion at the light entrance end of the second optical amplificationmedium is preferably less than 40%. Also, in the optical waveguideregion of each of the first and second optical amplification media, inorder to reduce the rare earth element concentration, a trivalent cationother than a rare earth element is preferably contained. In these cases,photodarkening can be still more effectively suppressed.

In the optical amplification module according to the present invention,the first optical amplification medium preferably has a core, an innercladding provided on an outer periphery of the core region, and an outercladding provided on an outer periphery of the inner cladding. The coreregion allows a single mode propagation of the light to be amplifiedentering it. The inner cladding allows a multi-mode propagation of thefirst pumping light outputted from the first pumping section. The secondoptical amplification medium preferably has a core region, an innercladding provided on an outer periphery of the core region, and an outercladding provided on an outer periphery of the inner cladding. The coreregion allows a single mode propagation of the first amplified lightentering it. The inner cladding allows a multi-mode propagation of thefirst pumping light having passed through the first opticalamplification medium. The doping concentration of the rare earth elementin each of the first and second optical amplification media, ispreferably 2000 wt.ppm or more. In these cases, still higheramplification gain is obtained while suppressing photodarkening.

Furthermore, the optical amplification module according to the presentinvention may comprises a plurality of optical amplification units eachhaving an pumping light source, an optical multiplexer and an opticalamplification medium. In this case, the optical amplification units maybe directly connected together by fusion-splicing, or they may beoptically connected via optical components such as an isolator and aband pass filter. More specifically, the optical amplification moduleaccording to the present invention comprises, at least, a first opticalamplification unit and second optical amplification unit.

The first optical amplification unit amplifies light which enters it,and emits a first amplified light. The first optical amplification unitspecifically comprises a first optical amplification medium, a firstpumping light source and a first optical multiplexer. The first opticalamplification medium has an optical waveguide region which is doped witha rare earth element. The first optical amplification medium has a lightentrance end into which light enters, and a light exit end which emits afirst amplified light that has been amplified inside the first opticalamplification medium. The first pumping light source outputs a firstpumping light of a predetermined wavelength. The first opticalmultiplexer multiplexes the light to be amplified together with thefirst pumping light. The first optical multiplexer is disposed on thelight entrance end side of the first optical amplification medium suchthat the first pumping light propagates in the order from the firstoptical amplification medium to second optical amplification medium. Thesecond optical amplification unit further amplifies the first amplifiedlight emitted from the first optical amplification unit, and emits asecond amplified light. The second optical amplification unitspecifically comprises a second optical amplification medium, a secondpumping light source and a second optical multiplexer. The secondoptical amplification medium has an optical waveguide region which isdoped with a rare earth element in a higher concentration than the rareearth element concentration of the first optical amplification medium.The second optical amplification medium has a light entrance end intowhich the first amplified light enters, and a light exit end which emitsthe second amplified light that has been amplified inside the secondoptical amplification medium. The second pumping light source outputs asecond pumping light with a wavelength different from that of the firstpumping light. The second optical multiplexer multiplexes the firstamplified light emitted from the first optical amplification medium andthe first pumping light, together with the second pumping light. Thesecond optical multiplexer is disposed between the first opticalamplification medium and the second optical amplification medium. Asdescribed above, in the case that the optical amplification module isconstituted by the plurality of optical amplification units, thewavelength of the first pumping light is preferably included in the 915nm wavelength band or 940 nm wavelength band. On the other hand, thewavelength of the second pumping light is preferably included in the 974nm wavelength band. In this specification, the pumping light in the 915nm wavelength band means light having a wavelength range of 120 nmcentered on a wavelength of 915 nm. The pumping light in the 940 nmwavelength band means light having a wavelength range of ±20 nm centeredon a wavelength of 940 nm. Further, the pumping light in the 974 nmwavelength band means light having a wavelength range of +5 nm centeredon a wavelength of 974 nm.

In addition to the first and second optical amplification units, theoptical module according to the present invention may comprise a thirdoptical amplification unit. In this case, the third opticalamplification unit further amplifies the second amplified light emittedfrom the second optical amplification unit, and emits a third amplifiedlight. The third optical amplification unit specifically comprises athird optical amplification medium, a third pumping light source and athird optical multiplexer. The third optical amplification medium has anoptical waveguide region which is doped with a rare earth element in ahigher concentration than the rare earth element concentration of thefirst optical amplification medium. The third optical amplificationmedium has a light entrance end into which the second amplified lightenters, and a light exit end which emits the third amplified light thathas been amplified inside the third optical amplification medium. Thethird pumping light source outputs a third pumping light with awavelength different from that of the first pumping light. The thirdoptical multiplexer multiplexes the third pumping light together withthe second amplified light emitted from the second optical amplificationmedium, the first pumping light and the second pumping light. The thirdoptical multiplexer is disposed between the second optical amplificationmedium and the third optical amplification medium. This third opticalamplification unit has a substantially identical construction to that ofthe aforesaid second optical amplification unit. Therefore, thewavelength of the third pumping light is included in the 974 nmwavelength band.

The laser light source according to the present invention comprises alight source for outputting light to be amplified, and an opticalamplification module having the aforesaid construction which amplifiesthe light outputted from the light source. In this laser light source,the light outputted from the light source is amplified in the opticalamplification module, and the amplified light is outputted.

The invention will be more fully understood from the detaileddescription given hereinbelow and the accompanying drawings, which aregiven by way of illustration only and are not to be considered aslimiting the present invention.

Further scope of applicability of the invention will become apparentfrom the detailed description given hereinafter. However, it should beunderstood that the detailed description and specific examples, whileindicating preferred embodiments of the invention, are given by way ofillustration only, since various changes and modifications within thespirit and scope of the invention will be apparent to those skilled inthe art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a constriction of a first embodiment of alaser light source according to the present invention (including anoptical amplification module according to the present invention);

FIG. 2 is a cross-sectional diagram showing the construction of each ofa first optical amplification medium and second optical amplificationmedium which can be used in the optical amplification module accordingto the present invention, and a refractive index profile thereof;

FIG. 3 is a diagram showing the construction of a laser light sourceaccording to a comparative example;

FIG. 4 is a graph showing the variation of population inversion in thelongitudinal direction of an optical amplification medium included inthe laser light source according to the comparative example;

FIG. 5 is a graph showing the variation of population inversion in thelongitudinal direction of the first optical amplification medium andsecond optical amplification medium used in the optical amplificationmodule of the laser light source according to the first embodiment;

FIG. 6 is a graph showing the wavelength dependency of the absorptioncross-section and emission cross-section in a Yb-doped fiber as anexample of the first and second optical amplification media;

FIG. 7 is a graph showing the variation of population inversion in thelongitudinal direction of the first optical amplification medium andsecond optical amplification medium used in the optical amplificationmodule of the laser light source according to the first embodiment;

FIG. 8 is a graph showing the relation between a length and non-linearshift amount for the second optical amplification medium used in theoptical amplification module of the laser light source according to thefirst embodiment;

FIG. 9 is a drawing showing the construction of a second embodiment of alaser light source according to the present invention (including anoptical amplification module according to the present inventioncomprising plural optical amplification units); and

FIG. 10 is a graph comparatively describing the effects of the firstembodiment and the second embodiment of the laser light source accordingto the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of an optical amplification module andlaser light source according to the present invention will be explainedin detail with reference to FIGS. 1 to 10. In the explanation of thedrawings, constituents identical to each other will be referred to withnumerals identical to each other without repeating their overlappingdescriptions.

First Embodiment

FIG. 1 is a drawing showing the construction of a first embodiment of alaser light source according to the present invention. A laser lightsource 1 shown in FIG. 1 comprises a light source 10 and an opticalamplification module 20 (optical amplification module according to thepresent invention). In the laser light source 1, the light from thelight source 10 is amplified in the optical amplification module 20, andthe obtained amplified light is emitted.

The optical amplification module 20 comprises a first opticalamplification medium 21, a second optical amplification medium 22 and apumping section 23. Each of the first optical amplification medium 21and the second optical amplification medium 22 is an optical waveguidewhose optical waveguide region is doped with a rare earth element, andis preferably an optical fiber having a core region doped with Yb as arare earth element. One end of the first optical amplification medium 21and the second optical amplification medium 22, respectively, areoptically connected, and preferably fusion-spliced. The rare earthelement concentration of the second optical amplification medium 22 ishigher than the rare earth element concentration of the first opticalamplification medium 21.

The pumping section 23 includes a pumping light source 24 and an opticalcoupler (optical multiplexer) 25. The pumping light source 24 outputs awavelength which can pump the rare earth element included in each of thefirst optical amplification medium 21 and the second opticalamplification medium 22. The pumping light source 24 is preferably, forexample, a laser diode or the like. The optical coupler 25 outputs thelight to be amplified from the light source 10 to the first opticalamplification medium 21, and also outputs the pumping light inputtedfrom the pumping light source 24 to the first optical amplificationmedium 21.

In this optical amplification module 20, the pumping light outputtedfrom the pumping light source 24 passes through the optical coupler 25,and then propagates in the order from the first optical amplificationmedium 21 to second optical amplification medium 22. The light to beamplified from the light source 10 is also inputted into the opticalamplification module 20, and after passing through the optical coupler25, is amplified in the first optical amplification medium 21 and theoptical amplification medium 22, respectively. The obtained amplifiedlight is outputted from the optical amplification module 20.

As described above, in this first embodiment, the pumping lightoutputted from the pumping section 23 first propagates through the firstoptical amplification medium 21, and then propagates through the secondoptical amplification medium 22. In the first optical amplificationmedium 21 through which the pumping light propagates first, although thepower of the pumping light which propagates therethrough is relativelyhigh, the rare earth element concentration is relatively low. On theother hand, in the second optical amplification medium 22 through whichthe pumping light propagates next, although the rare earth elementconcentration is relatively high, the power of the pumping light whichpropagates therethrough is relatively low. Due to this, the populationinversion of the first optical amplification medium with a low rareearth element concentration can be increased, while the populationinversion of the second optical amplification medium with a high rareearth element concentration can be decreased. Therefore, in both thefirst optical amplification medium 21 and the second opticalamplification medium 22, a high amplification gain can be obtained whilephotodarkening is suppressed.

The population inversion in the first optical amplification medium 21 ispreferably 40% or more, and the rare earth element concentration in thefirst optical amplification medium 21 is preferably not greater thanhalf of the rare earth element concentration in the second opticalamplification medium 22. The unsaturated absorption of the first opticalamplification medium 21 is preferably not less than 60% of theunsaturated absorption of the second optical amplification medium 22.The population inversion at the light entrance end of the second opticalamplification medium 22 (end face of the second optical amplificationmedium 22 fusion-spliced to the light exit end of the first opticalamplification medium 21), is preferably less than 40%. Also, in order toreduce the rare earth element concentration, a trivalent cation otherthan a rare earth element, such as Al³⁺ or the like, is preferably alsoadded to the optical waveguide region of each of the first opticalamplification medium 21 and the second optical amplification medium 22.In these cases, photodarkening may be suppressed still more effectively.

FIG. 2 is a cross-sectional view showing the construction of each of thefirst optical amplification medium 21 and the second opticalamplification medium 22, and a refractive index profile thereof. Thearea (a) of FIG. 2 shows a cross-section which intersectsperpendicularly with the optical axis AX, and the area (b) is arefractive index profile in the radial direction which intersectsperpendicularly with this optical axis AX. As shown in the area (a),preferably, each of the first optical amplification medium 21 and secondoptical amplification medium 22 has a core 201 allowing a single modepropagation of light to be amplified and doped with a rare earthelement, an inner cladding 202 provided on an outer periphery of thecore 201 and allowing a multi-mode propagation of the pumping light, andan outer cladding 203 provided on an outer periphery of the innercladding 202. The core 201 has a refractive index n1, the inner cladding202 has a refractive index n2 lower than that of the core 201, and theouter cladding 203 has a refractive index n3 lower than that of theinner cladding 202. The rare earth element concentration of each of thefirst optical amplification medium 21 and the second opticalamplification medium 22 is preferably 2000 wt.ppm or more. In this case,photodarkening is suppressed, and the light to be amplified can beamplified with still higher gain. The trivalent cation other than a rareearth element, such as Al³⁺ or the like, may also be added to the core201.

In a refractive index profile 250 shown in the area (b) of FIG. 2, aregion 251 indicates the refractive index in the radial direction of thecore 201, a region 252 indicates the refractive index in the radialdirection of the inner cladding 202, and a region 253 indicates therefractive index in the radial direction of the outer cladding 203.

FIG. 3 is a diagram showing the construction of the laser light sourceaccording to a comparative example. A laser light source 9 according tothe comparative example, shown in this FIG. 3, comprises the lightsource 10 for outputting light to be amplified, and an opticalamplification module 20A. In the laser light source 9 according to thecomparative example, the light to be amplified from the light source 10is amplified in the optical amplification module 20A, and the obtainedamplified light is emitted. The optical amplification module 20A has aconstruction wherein the first optical amplification medium 21 isomitted from the optical amplification module 20 (optical amplificationmodule according to the present invention) used in the laser lightsource 1 of the aforesaid first embodiment.

The action and effect of the laser light source 1 according to the firstembodiment will now be described in comparison with the laser lightsource 9 according to the comparative example. The first opticalamplification medium 21 and the second optical amplification medium 22are both optical fibers each having the structure shown in FIG. 2, andthe core 201 is doped with Yb as a rare earth element. The wavelength ofthe pumping light is 915 nm and the wavelength of the light to beamplified (seed beam) is 1064 nm. The Yb concentration of the secondoptical amplification medium 22 is 15000 wt.ppm. FIGS. 4 and 5 aregraphs measured under such conditions.

FIG. 4 is a graph showing the variation of population inversion in thelongitudinal direction of the optical amplification medium 22 in thelaser light source 9 according to the comparative example. In FIG. 4,the horizontal axis represents the pumping light propagation distancefrom the optical coupler 25. As shown in this FIG. 4, the populationinversion is higher the closer to the light entrance end of the opticalamplification medium 22, and its degree of amplification is also larger.Therefore, in the laser light source 9 according to this comparativeexample, photodarkening tends to occur easily.

FIG. 5 is a graph showing the variation of population inversion in thelongitudinal direction of the first optical amplification medium 21 andthe second optical amplification medium 22 used in the laser lightsource 1 of the first embodiment. In FIG. 5, the horizontal axisrepresents the pumping light propagation distance from the opticalcoupler 25. In this measurement, the pumping light power is constantsuch that the same output power is obtained as in the aforesaidcomparative example. The rare earth doping concentration and length L1of the first optical amplification medium 21, and the length L2 of thesecond optical amplification medium 22, are adjusted such that the sameunsaturated absorption is obtained as in the aforesaid comparativeexample. Also, the rare earth element concentration of the first opticalamplification medium 21 is half of the rare earth element concentrationof the second optical amplification medium 22 (i.e., the rare earthelement concentration of the first optical amplification medium 21 is7500 wt.ppm).

In FIG. 5, the curve G510 shows the population inversion when the totallength of the first and second optical amplification media 21, 22 wasvaried and the length L2 of the second optical amplification medium 22was fixed to 9 m, the curve G520 shows the population inversion when thetotal length of the first and second optical amplification media 21, 22was varied and the length L2 of the second optical amplification medium22 was fixed to 8 m, the curve G530 shows the population inversion whenthe total length of the first and second optical amplification media 21,22 was varied and the length L2 of the second optical amplificationmedium 22 was fixed to 7 m, the curve G540 shows the populationinversion when the total length of the first and second opticalamplification media 21, 22 was varied and the length L2 of the secondoptical amplification medium 22 was fixed to 6 m, and the curve G550shows the population inversion when the total length of the first andsecond optical amplification media 21, 22 was varied and the length L2of the second optical amplification medium 22 was fixed to 5 m.

It can be seen that, when the length L2 of the second opticalamplification medium 22 is 5 m to 6 m as shown in this FIG. 5, thepopulation inversion at the pumping light entrance end of the secondoptical amplification medium 22 is about 40% to 50%, which issufficiently low. Therefore, in the laser light source 1 according tothe first embodiment, photodarkening can be suppressed.

FIG. 6 is a graph showing the wavelength dependency of the absorptioncross-section and emission cross-section in a Yb-doped fiber as anexample of the first optical amplification medium 21 and second opticalamplification medium 22. In FIG. 6, the curve G610 indicates anabsorption cross-section (m²) for light with a wavelength of 850 nm to1150 nm, and the curve G620 indicates an emission cross-section (m²) forlight with a wavelength of 850 nm to 1150 nm. As can be seen from FIG.6, all the curves have a peak near a wavelength of 974 nm. The Inventorstherefore set the pumping light wavelength to 974 nm and the pumpinglight power to be the same as the aforesaid pumping light power,adjusted the length L1 of the first optical amplification medium 21 andthe length L2 of the second optical amplification medium 22 to obtainthe same output power as the aforesaid output power, and measured therelation between the length of the optical amplification media and thepopulation inversion.

As a result, it was found that when only the second opticalamplification medium 22 was used without using the first opticalamplification medium 21 (as in the laser light source 9 of thecomparative example), the length L2 of the second optical amplificationmedium 22 was about 6 m, which was sufficient. This is because in theoptical amplification medium to which Yb was added, the absorptioncoefficient is larger at a wavelength of 974 nm than at a wavelength of915 nm, so a shorter length is sufficient.

FIG. 7 shows a graph corresponding to variation of the populationinversion in the longitudinal direction for the first opticalamplification medium 21 and the second optical amplification medium 22used in the laser light source 1 according to the first embodiment. Inmeasuring this variation of population inversion, the pumping lightwavelength was 974 nm and the first optical amplification medium 21 hada low concentration. Also, in this measurement, while adjusting thelength of the first optical amplification medium 21 and second opticalamplification medium 22, the total unsaturated absorption of the firstoptical amplification media 21 and second optical amplification medium22 was adjusted. In FIG. 7, the curve G710 indicates the populationinversion when the total length of the first and second opticalamplification media 21, 22 was varied and the length L2 of the secondoptical amplification medium 22 was fixed to 6 m, the curve G720indicates the population inversion when the total length of the firstand second optical amplification media 21, 22 was varied and the lengthL2 of the second optical amplification medium 22 was fixed to 5 m, thecurve G730 indicates the population inversion when the total length ofthe first and second optical amplification media 21, 22 was varied andthe length L2 of the second optical amplification medium 22 was fixed to4.5 m, the curve G740 indicates the population inversion when the totallength of the first and second optical amplification media 21, 22 wasvaried and the length L2 of the second optical amplification medium 22was fixed to 4 m, the curve G750 indicates the population inversion whenthe total length of the first and second optical amplification media 21,22 was varied and the length L2 of the second optical amplificationmedium 22 was fixed to 3.5 m, and the curve G760 indicates thepopulation inversion when the total length of the first and secondoptical amplification media 21, 22 was varied and the length L2 of thesecond optical amplification medium 22 was fixed to 3 m.

As a result, it can be seen that, when the length L2 of the secondoptical amplification medium 22 was 3.5 m and the total unsaturatedabsorption of the first optical amplification medium 21 and secondoptical amplification medium 22 was adjusted, the population inversionat the pumping light entrance end of the second optical amplificationmedium 22 was about 40% at a length of 3.5 m, which is sufficiently low.Hence, to suppress photodarkening, the use of pumping light having awavelength of 974 nm decreases the population inversion, and is thusmore effective.

In the optical amplification module 20 of the laser light source 1according to this first embodiment, the low concentration of the firstoptical amplification medium 21 has an identical significance to thelengthening of the first and second optical amplification media 21, 22.Hence, in the first and second optical amplification media 21, 22, thenonlinear shift amount becomes large. FIG. 8 is a graph showing therelation of the length L2 of the second optical amplification mediumused in the optical amplification module 20 of the laser light source 1according to this first embodiment, and the nonlinear shift amount. Whenthe wavelength of the pumping light is 974 nm and the length L2 of thesecond optical amplification medium 22 is 3.5 m, the nonlinear shiftamount can be suppressed to about 1 dB or less than the nonlinear shiftamount of the optical amplification medium (FIG. 3) in the comparativeexample. Since this value is determined to be within the limits of theloss variation at the fusion-spliced portion between the first opticalamplification medium 21 and the second optical amplification medium 22,and the loss variation from the second optical amplification medium 22to the output end of the laser light source 1, the increasing nonlinearshift amount due to use of the first optical amplification medium 21, isnot a problem.

Second Embodiment

FIG. 9 is a drawing showing a construction of a second embodiment of thelaser light source according to the present invention. A laser lightsource 2 according to the second embodiment shown in this FIG. 9comprises the light source 10 for outputting light to be amplified, andan optical amplification module 30 (optical amplification moduleaccording to the present invention). In the laser light source 2, thelight to be amplified from the light source 10 is amplified in theoptical amplification module 30, and the obtained amplified light isoutputted.

The optical amplification module 30 may comprise two or more opticalamplification units, each comprising a pumping light source, an opticalmultiplexer and an optical amplification medium. In the laser lightsource 2 according to the second embodiment, the optical amplificationmodule 30 comprises a first optical amplification unit 30A, a secondoptical amplification unit 30B and a third optical amplification unit30C. The first to third optical amplification units 30A to 30C areoptically connected via isolators 31, 32, but the light exit end of theoptical amplification unit of the first stage and the light entrance endof the optical amplification unit of the last stage, may be directlyfusion-spliced together.

The first optical amplification unit 30A comprises a first opticalamplification medium 21 and a first pumping section 23A. The firstoptical amplification medium 21, as described above, is an opticalwaveguide whose optical waveguide region is doped with the rare earthelement as described above, and is preferably an optical fiber whosecore region is doped with Yb as a rare earth element.

The first pumping section 23A includes a first pumping light source 24Aand a first optical coupler (optical multiplexer) 25A. The first pumpinglight source 24A outputs a first pumping light having a wavelength whichcan pump the rare earth elements added to the first opticalamplification medium 21, specifically a wavelength in the 915 nmwavelength band or the 940 nm wavelength band. The pumping light in the915 nm wavelength band means light having a wavelength range of ±20 nmcentered on 915 nm. The pumping light in the 940 nm wavelength bandmeans light having a wavelength range of ±20 nm centered on 940 nm. Thisfirst pumping light source 24A is preferably, for example, a laser diodeor the like.

The first optical coupler 25A outputs the light to be amplified from thelight source 10 to the first optical amplification medium 21, and alsooutputs the first pumping light inputted from the first pumping lightsource 24A to the first optical amplification medium 21.

In this first optical amplification unit 30A, the first pumping lightoutputted from the first pumping light source 24A propagates through thefirst optical amplification medium 21 via the first optical coupler 25A.The light to be amplified from the light source 10 is also inputted intothe first optical amplification unit 30A, and the incident light to beamplified is amplified in the first optical amplification medium 21after passing through the first optical coupler 25A. The obtained firstamplified light from the first optical amplification unit 30A isoutputted to the second optical amplification unit 30B.

The second optical amplification unit 30B comprises a second opticalamplification medium 22 and a second pumping section 23B. The secondoptical amplification medium 22, as described above, is also an opticalwaveguide whose optical waveguide region is doped with a rare earthelement, and is preferably, an optical fiber whose core region is dopedwith Yb as a rare earth element. However, the rare earth elementconcentration of this second optical amplification medium 22 is higherthan the rare earth element concentration of the aforesaid first opticalamplification medium 21.

The second pumping section 23B includes a second pumping light source24B and a second optical coupler (optical multiplexer) 25B. The secondpumping light source 24B outputs a second pumping light having awavelength which can pump the rare earth element added to the secondoptical amplification medium 22, i.e., a wavelength of 974 nm. Thepumping light in the 974 nm wavelength band means light having awavelength range of ±5 nm centered on 974 nm. This second pumping lightsource 24B is preferably, for example, a laser diode or the like. Thesecond optical coupler 25B outputs the first amplified light whicharrived from the first optical amplification unit 30A via the isolator31 to the second optical amplification medium 22, and also outputs thesecond pumping light inputted from the second pumping light source 24Bto the second optical amplification medium 22.

In this second optical amplification unit 30B, the second pumping lightoutputted from the second pumping light source 24B propagates throughthe second optical amplification medium 22 via the second opticalcoupler 25B. The first amplified light which arrived from the firstoptical amplification unit 30A via the isolator 31 is also inputted intothe second optical amplification unit 30B, and the first amplified lightis amplified in the second optical amplification medium 22 after passingvia the second optical coupler 25B. The obtained second amplified lightis outputted from the second optical amplification unit 30B to the thirdoptical amplification unit 30C.

The third optical amplification unit 30C disposed after the secondoptical amplification unit 30B has a substantially identicalconstruction to that of the second optical amplification unit 30B. Thisthird optical amplification unit 30C specifically comprises the secondoptical amplification medium 22 and a third pumping section 23C. Thesecond optical amplification medium 22 used in this third opticalamplification unit 30C is also an optical waveguide whose opticalwaveguide region is doped with a rare earth element as described above,and is preferably an optical fiber whose core region is doped with Yb asa rare earth element. However, the rare earth element concentration ofthis second optical amplification medium 22 is higher than the rareearth element concentration of the aforesaid first optical amplificationmedium 21.

The third pumping section 23C includes a third pumping light source 24Cand a third optical coupler (optical multiplexer) 25C. As in the case ofthe second pumping light source 24B, the third pumping light source 24Coutputs a third pumping light having a wavelength which can pump therare earth elements added to the second optical amplification medium 22,i.e., a wavelength of 974 nm. This third pumping light source 24C ispreferably, for example, a laser diode or the like. The third opticalcoupler 25C outputs the second amplified light which arrived from thesecond optical amplification unit 30B via the isolator 32 to the secondoptical amplification medium 22, and also outputs the third pumpinglight inputted from the third pumping light source 24C to the secondoptical amplification medium 22.

In the third optical amplification unit 30C, the third pumping lightoutputted from the third pumping light source 24C propagates through thesecond optical amplification medium 22 via the third optical coupler25C. The second amplified light which arrived from the second opticalamplification unit 30B via the isolator 32 is also inputted to the thirdoptical amplification unit 30C, and the second amplified light isamplified in the second optical amplification medium 22 after passingthrough the third optical coupler 25C. The obtained third amplifiedlight is outputted from the third optical amplification unit 30C as thefinal output of the laser light source 2 according to the secondembodiment.

Next, referring to FIG. 10, the effect of the first embodiment andsecond embodiment using the laser light source according to the presentinvention will be explained. FIG. 10 is a graph for describing, incomparative terms, the effect of the first embodiment and secondembodiment using the laser light source according to the presentinvention. In the laser light source 1 according to the firstembodiment, the optical amplification module 20 has a one-stageconstruction wherein the first optical amplification medium 21 and thesecond optical amplification medium 22 are fusion-spliced together. Onthe other hand, in the laser light source 2 according to the secondembodiment, the optical amplification module 30 has a two-stageconstruction comprising the first optical amplification unit 30Aincluding the first optical amplification medium 21 and the secondoptical amplification unit 30B including the second opticalamplification medium 22. In the one-stage optical amplification module20 of the first embodiment, light passing through the first and secondoptical amplification media 21, 22 is amplified by pumping light havinga wavelength of 915 nm. The length of the first optical amplificationmedium 21 is 6 m, and the length of the second optical amplificationmedium 22 is 6 m. Also, the rare earth element concentration of thefirst optical amplification medium 21 is half that of the second opticalamplification medium 22. On the other hand, the two-stage optical module30 of the second embodiment comprises the first optical amplificationunit 30A and second optical amplification unit 30B, and in the firstoptical amplification unit 30A of the first stage, the wavelength of thefirst pumping light is 915 nm and the length of the first opticalamplification medium 21 is 6 m. In the second optical amplification unit30B of the second stage, the wavelength of the second pumping light is974 nm, and the length of the second optical amplification medium 22 is6 m. In FIG. 10, the curve 1010 indicates the variation of thepopulation inversion in the one-stage optical amplification module 20,and corresponds to the curve G540 in FIG. 5. Also, the curve G1020indicates the variation of the population inversion in the two-stageoptical amplification module 30.

As can be seen from FIG. 10, in the two-stage optical amplificationmodule 30, although the population inversion at the pumping lightentrance end of the first optical amplification medium 21 (in the firstoptical amplification unit 30A) is about 90%, the population inversionat the exit end of the first optical amplification medium 21 is about50%. Next, the first amplified light (amplified light outputted from thefirst optical amplification unit 30A) which passed through the isolator31 and second optical coupler 25B in that order propagates through thesecond optical amplification medium 22. By adjusting the second pumpinglight power which enters the second optical amplification medium 22, thepopulation inversion of the second optical amplification medium 22 canbe adjusted. In particular, in this measurement, the first pumping lightpower used in the first optical amplification unit 30A is 42.5 dB, andthe second pumping light power used in the second optical amplificationunit 30B is 36.5 dB.

As compared with the laser light source 1 according to the firstembodiment including the one-stage optical amplification module 20 whichis constituted by the first optical amplification medium 21 and secondoptical amplification medium 22 directly fusion-spliced together, in thecase of the second laser light source 2 including the two-stage opticalamplification module 30, there is scatter in the population inversion inthe longitudinal direction of the second optical amplification medium22, but the population inversion in the second optical amplificationmedium 22 is suppressed sufficiently low, so photodarkening can also besufficiently suppressed. Therefore, also in the laser light source 2according to the second embodiment, the same effect as that of the laserlight source 1 of the first embodiment is obtained.

The invention is not limited to the above embodiments, variousmodifications being possible within the scope and spirit of the appendedclaims. For example, although in the aforesaid embodiments, forwardpumping wherein the pumping light propagates in the same direction asthe propagation direction of the light to be amplified was used,backward pumping wherein the pumping light propagates in the oppositedirection to the propagation direction of the light to be amplified, mayalso be used.

As described above, in accordance with the present invention,photodarkening is effectively suppressed.

From the invention thus described, it will be obvious that theembodiments of the invention may be varied in many ways. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to one skilledin the art are intended for inclusion within the scope of the followingclaims.

1-10. (canceled)
 11. An optical amplification module, comprising: afirst optical amplification unit which amplifies an incident light to beamplified and emits a first amplified light, said first opticalamplification unit comprising: a first optical amplification mediumhaving an optical waveguide region which is doped with a rare earthelement, and having a light entrance end on which the light to beamplified is incident and a light exit end which emits a first amplifiedlight that has been amplified inside said first optical amplificationmedium; a first pumping light source for outputting a first pumpinglight of a predetermined wavelength; and a first optical multiplexer formultiplexing the light to be amplified and the first pumping light, saidmultiplexer being disposed on the light entrance end side of said firstoptical amplification medium such that the first pumping lightpropagates in the order from said first optical amplification medium tosaid second optical amplification medium; and a second opticalamplification unit which further amplifies the first amplified lightemitted from said first optical amplification unit and emits a secondamplified light, said second optical amplification unit comprising: asecond optical amplification medium having an optical waveguide regionwhich is doped with a rare earth element in a higher concentration thanthe rare earth concentration of said first optical amplification medium,and having a light entrance end on which the first amplified light isincident and a light exit end which emits a second amplified light thathas been amplified inside said second optical amplification medium; asecond pumping light source for outputting a second pumping light ofdifferent wavelength from the first pumping light; and a second opticalmultiplexer for multiplexing the first amplified light outputted fromsaid first optical amplification medium and the first pumping lighttogether with the second pumping light, said optical multiplexer beingdisposed between said first optical amplification medium and said secondoptical amplification medium.
 12. An optical amplification moduleaccording to claim 11, wherein the wavelength of said first pumpinglight is included within one of the 915 wavelength band and the 940 nmwavelength band, and wherein the wavelength of said second pumping lightis included within the 974 m wavelength band.
 13. An opticalamplification module according to claim 1, further comprising: a thirdoptical amplification unit which further amplifies the second amplifiedlight outputted from said second optical amplification unit, said thirdoptical amplification unit comprising: a third optical amplificationmedium having an optical waveguide region which is doped with a rareearth element in a higher concentration than the rare earthconcentration of said first optical amplification medium, and having alight entrance end on which the second amplified light is incident and alight exit end which emits a third amplified light that has beenamplified inside said third optical amplification medium; a thirdpumping light source for outputting a third pumping light of differentwavelength from the wavelength of the first pumping light; and a thirdoptical multiplexer for multiplexing the second amplified lightoutputted from said second optical amplification medium, the firstpumping light and the second pumping light together with the thirdpumping light, said optical multiplexer being disposed between saidsecond optical amplification medium and said third optical amplificationmedium.
 14. An optical amplification module according to claim 13,wherein the wavelength of said third pumping light is included withinthe 974 nm wavelength band.
 15. A laser light source, comprising: alight source for outputting light to be amplified; and an opticalamplification module according to claim 11, said optical amplificationmodule amplifying the light to be amplified outputted from said lightsource.